Helicopters, tiltrotor, and other vertical take-off aircraft (hereinafter collectively referred to as "aircraft") are often called upon to move large cargo into areas that are not readily accessible to land vehicles. When moving particularly large or heavy pieces of cargo, the cargo is usually suspended beneath the aircraft by a set of slings that connect the load created by the cargo to one or two attachment points located on the bottom of the aircraft. While the number of the attachment points and the design of the attachment points may vary, they typically take the form of a hook from which the cargo is suspended. The hooks are remotely operable to allow the cargo that is being carried to be released from the cockpit. Suspending the cargo beneath the helicopter allows the cargo to be quickly loaded or unloaded without forcing the aircraft to land.
When ferrying a heavy load beneath an aircraft at high speeds, it has been found to be advantageous to suspend the cargo from two attachment points rather than a single attachment point. A dual-point suspension distributes the weight of the cargo more evenly, allowing heavier cargo to be carried by the aircraft. A dual-point suspension that has a first attachment point positioned at the front of the aircraft and a second attachment point positioned at the rear of the aircraft also provides directional stability when flying. Keeping the cargo oriented parallel with the aircraft's direction of travel reduces the tendency of the cargo to spin or twist during flight.
While it is desirable to carry cargo suspended from two attachment points, a dual-point suspension generates problems that are not present with a single attachment point. When a component of a single-point suspension system fails, the aircraft is typically not placed in any danger. For example, a failure of a sling connecting cargo to a single attachment point on an aircraft would merely drop the load to the ground. Although the cargo would be lost, the failure of the single-point suspension system would not generally pose a danger to the aircraft.
In contrast, a failure in a dual-point suspension system may potentially harm the aircraft that is carrying cargo. At least two different types of failures may arise when cargo is carried by a dual-point suspension. A component of the suspension system itself may fail. For example, a hook may break or a sling from one of the hooks may become severed or disconnected from the cargo. A failure of one of the attachment points transfers the entire cargo load onto the sole remaining hook. Alternatively, the cargo being carried may break into several different pieces due to the rigors of flight or by collision with another object. Cargo breaking into pieces results in a portion of the original cargo being suspended from each hook. If the cargo breaks into substantially unequal pieces, the load on one hook will lessen, while the load on the other hook will increase. Either type of suspension system failure could potentially harm the ferrying aircraft. Portions of the cargo may swing up and physically strike the aircraft, or the weight shift caused by a failure of an attachment point could induce an instability in the flight characteristics of the aircraft. When a component of a dual-point suspension system fails, or when cargo breaks into pieces, it is therefore the best course of action to drop the cargo as quickly as possible. Jettisoning the cargo is the preferred alternative to potentially damaging the aircraft carrying the cargo.
Accurately determining when a portion of a dual-point suspension system fails is therefore critical to carrying any cargo by a two point suspension. Previous systems for determining when a dual-point suspension system fails rely on detecting when the load on a hook falls below a minimum value for a period of time. To detect the load placed on each hook, load cells are typically built into the hooks. The load cells control the magnitude of an electrical signal such that the magnitude is proportional to the amount of the load supported by the hook. The load cell signals are periodically sampled in order to provide an accurate measure of the amount of the load supported by each hook at a given point in time. Prior art systems use the load cell data to determine when a hook load drops below a minimum value for a short period of time. When this occurs, the cargo is jettisoned. For example, in one prior art system used on a CH53E Sea Stallion, if the load on a hook falls below 300 pounds for greater than 0.15 seconds, the cargo being carried is automatically jettisoned.
In ideal flight conditions, the prior art method of detecting a dual-point suspension failure and automatically jettisoning the cargo works reasonably well. However, false alarms and detection failures often occur in actual flight conditions. False alarms are typically created by the motion of the aircraft. Those skilled in the art will recognize that the load borne by each hook of a dual-point suspension system will rarely be constant during flight. The trajectory of the aircraft, including its speed, rate of turning, and rate of altitude gain or loss, will all affect the load supported by each hook of the suspension system. Turbulence will often cause the cargo to exert a varying force on each attachment point. When an aircraft maneuvers at low Gs or encounters turbulence, cargo may bounce, resulting in a false indication of a suspension point failure. The prior art system of detecting suspension failures therefore generates false alarms in certain circumstances.
The prior art system of detecting a dual-point suspension system failure also fails to detect many actual failures. Detection failures can occur when cargo suspended from a dual-point suspension system breaks apart. If cargo separates into two pieces, leaving more than the minimum jettison load value hanging from each hook of the system, prior art systems will not detect the failure and, as a result, not jettison the cargo. In summary, detecting a dual-point suspension failure by determining when the loads on the hooks drop below a minimum value for a period of time has proven to be inadequate because false alarms are generated and failures are not always detected. The present invention is directed to providing a method and apparatus for detecting suspension or load failure that overcomes these disadvantages.